The present invention relates to a system and method of noise reduction. More particularly, it relates to a receiver for receiving packetized audio signals and providing an audio output with reduced noise and a method of reducing impulsive noise in packetized audio signals.
Audio signals such as voice, music or other audio signals can be transmitted over a wireless interface. The audio signals are typically transmitted in packets including a packet header and a packet payload. Each packet payload includes a separate portion of the audio signal. The packetized audio signals are decoded into frames of digital audio signals which are combined in the proper sequence at the receiving end to produce the audio output signal.
During the wireless transmission, transmission errors can occur which corrupt some of the payload packets. The corrupted payload packets, which include missing or incorrect signal information, are decoded to form corrupted frames which produce undesirable audible noise in the audio output signal generated at the receiving end. Due to the short duration of the frames, the noise produced by corrupted frames is often perceived as clicks or other types of short duration noise called impulsive noise.
Error protection has been traditionally applied to detect and correct transmission errors. Some conventional error protection schemes include an error correction process, known as channel coding, which adds redundancy bits to the packets before the transmission takes place. In the receiver, these additional bits are used for detecting transmission errors as well as reconstructing the original payload contents if the transmission errors are within certain limits.
Many wireless transmission standards have a fixed channel capacity because the gross bit rate of transmission is fixed. For applications using fixed bit rate standards, error correction typically involves a tradeoff between the amount of error protection and the payload throughput. Using more bits to increase the error protection reduces the bits available for the payload, thus decreasing the payload throughput of the wireless transmission. A reduction in payload throughput results in either the transmission of a lower quality audio signal or a reduction in the amount of audio information, such as for example, the number of voice channels, which is transmitted.
Conventional error protection schemes for fixed channel capacity transmission standards handle this tradeoff in a variety of ways. Some error protection schemes maximize the audio channels available by using error protection in the packet header, but not the payload. However, with these header-only schemes, if the header is not corrupted but the payload is, the corruption will not be detected and impulsive noise will be generated at the receiver.
Other conventional error protection schemes allow for adapting the amount of error correction to the channel conditions. When there is more interference on a channel, more bits are used for error correction. For example GSM channel coding for adaptive multi-rate speech codec, the more interference a channel has the more bits used for channel coding and the less used for audio coding. However, the channel conditions may change more quickly than these approaches can compensate for.
It is desirable to improve the quality of packetized audio information transmitted over a wireless interface. It is also desirable to reduce impact of transmission errors on transmitted audio and minimize the channel bandwidth used for channel coding.